A facsimile has a signal processing circuit for receiving a facsimile-use control signal that is sent through a telephone line, and also has a telephone set for receiving a sound signal through the same telephone line. Therefore, the facsimile is provided with a function that switches the telephone line automatically to connect an input signal to the telephone set when the input signal from the telephone line is a sound signal for a telephone call.
For this reason, the facsimile has a signal discrimination circuit for determining whether the input signal in question is a control signal for facsimile or a sound signal. Here, the control signal includes a calling tone (hereinafter, referred to as CNG signal) for calling the recipient or other signals. The circuit, for example, shown in FIG. 5 is known as such a signal discrimination circuit. In the signal discrimination circuit, an input signal, which has been amplified by an amplifier 11, is first filtered through band-pass filters 12 that have three kinds of frequency bands that are different from one another. Output signals from the respective band-pass filters 12 are subjected to waveform shaping in waveform shaping circuits 13. Then, the output signals are compared with respective threshold values in comparison circuits 14, and the presence or absence of a signal within each frequency band is detected through this comparison.
A control signal for facsimiles consists of a single tone having a single frequency that is determined for each control signal. For example, in the case of the CNG signal, a signal of 1100 Hz lasting for 0.5 sec. is sent at intervals of 3 seconds. Here, suppose that one of the frequency bands of the band-pass filters 12 is set to a frequency band of the single tone, while the other frequency bands of the band-pass filters 12 are set to frequency bands that are respectively higher and lower than the frequency band. If an input signal is a control signal, only the corresponding comparison circuit 14 detects the signal while the other two comparison circuits 14 do not detect the signal.
If an input signal is a sound signal, the signal is detected by all the comparison circuits 14 or a plurality of the comparison circuits 14 because the sound signal has comparatively wide frequency components. Therefore, a decision circuit 15 is installed in order to execute logical operations based on the results of the detection made by the comparison circuits 14, and it is determined whether a certain input signal is the control signal or the sound signal through the logical operations.
However, telephone lines tend to be affected by noise inclusions. Therefore, in the above-mentioned telephone line, white noise tends to enter the control signal of the facsimile. The white noise is a signal which is distributed in all the frequency bands.
Therefore, the signal discrimination circuit, upon receiving an input signal consisting of a control signal containing white noise, tends to fail in discriminating the control signal from the sound signal because the signal is detected by all the comparison circuits 14 or a plurality of the comparison circuits 14. For this reason, the signal discrimination circuit tends to make a misjudgement and switch the telephone line to the telephone set even in the case of receiving the control signal.
As described above, in the signal discrimination circuit, since it can not discriminate white noise from sound signals, there is a possibility that upon receiving an input signal consisting of a control signal containing white noise, it will make a misjudgement determining the signal as a sound signal; this raises a problem that is related to low reliability in operation.
Upon receiving a control signal consisting of a single tone, this problem makes it difficult to discriminate the control signal due to white noise contained therein. Therefore, this problem is not limited to facsimiles, and is commonly encountered in digital signal processing apparatuses such as data communications terminals that are connected to analog signal lines such as telephone lines.